Method for operating a urea-water solution metering system and motor vehicle using the system

ABSTRACT

A method for operating a device for providing a liquid reactant to an exhaust gas system. The device includes at least one tank and an injection nozzle connected to one another by a line. The line has a pump for feeding the reactant. A controllable return line connected between the pump and the injection nozzle leads from the line to the tank. A controller controls operation of the injection nozzle, the pump and the return line, so that the reactant is only circulated intermittently through the controllable return line. A motor vehicle using the method is also provided.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation, under 35 U.S.C. §120, of copending International Application No. PCT/EP2009/057257, filed Jun. 12, 2009, which designated the United States; this application also claims the priority, under 35 U.S.C. §119, of German Patent Application DE 10 2008 030 756.4, filed Jun. 27, 2008, the prior applications are herewith incorporated by reference in their entirety.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a method for operating a device for providing a liquid reactant, in particular a liquid urea-water solution, to an exhaust system. The device has at least one tank and an injection nozzle connected to one another through a line as well as a pump for conveying the reactant. A controllable return line is connected between the pump and the injection nozzle and leads from the line to the tank. The invention is used, in particular, in a dosing system provided in motor vehicles.

It is known to provide catalytic converters for selective catalytic reaction (SCR) for the purification of exhaust gases of mobile internal combustion engines such as, in particular, diesel engines, in order to reduce nitrogen oxides contained in the exhaust gas. The nitrogen oxides contained in the exhaust gas react and are reduced to form elemental nitrogen at the catalytic converter, which generally includes a honeycomb catalyst carrier body with a corresponding catalytically active coating. That process may be motivated, in particular, by adding a reactant or reducing agent (such as, for example, urea and/or ammonia) to the exhaust gas. If urea is introduced into the exhaust system, the urea can be directly pyrolyzed as a result of contact with the hot exhaust gas, and thereby produce the desired ammonia gas. It is, however, also possible for the urea to be conducted across a catalytic converter for the hydrolysis of the urea, in such a way that a conversion of the urea into ammonia takes place in a catalytically motivated manner. The ammonia thus provided then flows, thoroughly mixed together with the exhaust gas, into the SCR catalyst carrier body.

It emerges from the method described above, that the added quantity of reducing agent or reactant must be precisely coordinated with the current situation. It is necessary to take into consideration, in particular, the pollutants currently present in the exhaust gas and also the reactivity of the SCR catalytic converter. A situation in which too little reactant is added, in such a way that the desired conversion of the exhaust-gas pollutants does not take place to the required extent, should also be avoided. The addition of too great a quantity of the reactant is also undesirable because the reactant can then likewise flow through the SCR catalytic converter without being converted, and there is accordingly the risk of the reactant being dissipated to the environment. For that reason, measures are desirable which ensure a precise dosing of a predetermined quantity of the reactant at the desired time.

In the device described herein, a reactant reservoir is formed which, in particular, is at an elevated pressure, for example higher than 3 bar or even at least 7 bar or 10 bar, through the use of a pump in a (flexible) line. At the desired time, the injection nozzle is then opened, and the injection quantity is then determined from the opening time of the injection nozzle and the pressure in the line. It is evident therefrom that such an extremely simply constructed system is likewise sensitive to inclusions, impurities and other disturbances in the conveying line.

SUMMARY OF THE INVENTION

It is accordingly an object of the invention to provide a method for operating a urea-water solution metering system and a motor vehicle using the system, which overcome the hereinafore-mentioned disadvantages and at least partially solve the highlighted problems of the heretofore-known devices and methods of this general type. In particular, it is sought to specify a method in which reproducible conditions should be ensured in the line leading to the injection nozzle, and in which, in particular, a simple and fast restoration of the desired system during operation is made possible in the event of faults being detected in the system. This relates, in particular, to the avoidance or elimination of gas inclusions or frozen partial regions of the reactant.

With the foregoing and other objects in view there is provided, in accordance with the invention, a method for operating a device for providing a liquid reactant to an exhaust system. The method comprises supplying the liquid reactant in the device from at least one tank through a line to an injection nozzle leading to the exhaust system, conveying the liquid reactant through the line with a pump, returning the liquid reactant from the line to the tank through a controllable return line connected between the pump and the injection nozzle, controlling operation of the injection nozzle, the pump and the return line with a controller, and only intermittently circulating the reactant through the controllable return line.

It should be noted in this case that the return line is generally connected to the line through a valve which can be opened and/or closed at predefined times. The return line is conventionally used to enable a release of pressure in the line between the injection nozzle and the pump in a situation in which the adding process is ended. The reactant fed into the line at elevated pressure through the use of the pump can then flow back through the return line into the tank. As a result, after the shut-down of the device, reactant is present in the line at a pressure substantially corresponding to ambient pressure. This also makes it possible, in the event of freezing of the device, for the reactant to expand, in particular to increase in volume by 11%, such as is to be expected with a urea-water solution.

In this case, the return line is used for intermittent circulation. This means, in particular, that the valve is opened and the pump conveys reactant from the tank to the return line and therefore back into the tank. When the circulation is ended, the return line is closed again and the pressure upstream of the injection nozzle is built up again to the desired value, in such a way that only then is a situation present again in which a controlled injection is possible. Consequently, no injection can be carried out in the circulation phase.

The aim of the circulation is, in particular, to remove gas inclusions which have arisen in the line during a period of standstill or due to unfavorable operating conditions, and/or to liquefy still-frozen partial regions of the reactant in the line as a result of the liquid reactant flowing past them and/or friction. The desired result after the circulation is for a substantially uniform density of the reactant to be present in the line again, and accordingly for it to be possible to directly set the desired pressure conditions upstream of the injection nozzle.

In accordance with another mode of the invention, the reactant is circulated for a conveying time period which corresponds to at least two times the time required for the device to convey reactant from the tank through the controllable return line and back into the tank. This means that the line is flushed through at least twice with reactant freshly extracted from the tank.

In accordance with a further mode of the invention, the reactant is circulated for a conveying time period which corresponds to at most five times the time required for the device to convey reactant from the tank through the controllable return line and back into the tank. This means, in particular, that such circulation should not take place too frequently, that is to say is limited in this case to five flush-throughs. With five flush-throughs, the fault will be eliminated in the majority of cases, and furthermore a relatively large amount of time and energy will already have been expended.

In accordance with an added mode of the invention, the circulation of the reactant is initiated by using a device for identifying at least solid matter or gases in the reactant. The device for identifying at least solid matter or gases includes a sensor, in particular a pressure sensor, which can determine the pressure in the line or in and/or directly upstream of the injection nozzle. This means, in particular, that the pressure profile during the injection phase is monitored by that device and is compared with an expected pressure change. If it is now established that the expected pressure level is not attained in the case of an acting outlet pressure and a predefined opening time of the injection nozzle, this may indicate, in particular, that gas inclusions and/or solid matter are present in the reactant. Accordingly, it would then be necessary, in the event of such solid matter and/or gases being identified in the reactant, to initiate the circulation, which may take place directly through the use of the controller.

In accordance with an additional mode of the invention, the circulation of the reactant is initiated at the start of operation by using a device for identifying a tank state variable. This means, in particular, sensors which can determine the filling level in the tank. The device described herein is, in particular, sensitive during a tank filling process, that is to say when new reactant has been introduced into the tank. It is possible specifically during such a process for gas inclusions and the like to form. If it is thus detected by the device for identifying a tank state variable of the circulation that a tank filling process has just taken place, the circulation is initiated directly upon the start of operation of the device.

In accordance with yet another mode of the invention, the reactant is circulated with an increased conveying rate as compared to when it is conveyed for an injection. This means, in particular, that the pump operates with a conveying speed and conveying frequency which are respectively increased during the circulation phase as compared to during the phase in which the conveying pressure is built up in the direction of the closed injection nozzle. The respective high conveying speed and conveying frequency of the pump likewise promote the breakdown of gaseous or solid inclusions in the reactant.

In accordance with yet a further mode of the invention, it is very particularly preferable if liquid urea-water solution is conveyed. That means, in particular, that liquid urea-water solution is added through the injection nozzle into the exhaust system. Urea-water solution is also widely known under the product name AdBlue®.

With the objects of the invention in view, there is concomitantly provided a motor vehicle, comprising an internal combustion engine, an exhaust system defining an exhaust gas flow direction from the internal combustion engine and including an SCR catalytic converter and an injection location for injecting a liquid reactant into the exhaust system upstream of the SCR catalytic converter in the flow direction, as well as at least one device. The at least one device includes at least: a tank for the liquid reactant;

a controllable injection nozzle at the injection location; a line interconnecting the tank and the injection nozzle; a pump for conveying the reactant from the tank to the injection nozzle; a return line connected to the line at a location between the pump and the injection nozzle, the return line leading to the tank; a valve for controlling the return line; and a controller for controlling operation of the injection nozzle, the pump and the return line by carrying out the method according to the invention.

The motor vehicle is, in particular, a utility vehicle, that is to say, for example, a bus, a truck, a tractor or the like. The invention is used, in particular, in internal combustion engines in the form of a diesel engine.

Other features which are considered as characteristic for the invention are set forth in the appended claims, noting that the features specified individually in the claims may be combined with one another in any desired technologically meaningful way and form further structural variants of the invention.

Although the invention is illustrated and described herein as embodied in a method for operating a urea-water solution metering system and a motor vehicle using the system, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.

The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE of the drawing is a schematic and block diagram of a motor vehicle having a device for providing a liquid reactant to an exhaust system.

DETAILED DESCRIPTION OF THE INVENTION

Referring now in detail to the single FIGURE of the drawing for explaining the invention and the technical field in more detail by showing a particularly preferred structural variant to which the invention is not restricted, there is seen a schematic and block diagram of a motor vehicle 10 having a device 1. An internal combustion engine 11, for example in the form of a diesel engine, with an adjoining exhaust system 3, are illustrated in a left-hand part of the FIGURE. Exhaust gas produced in the internal combustion engine 11 flows in a flow direction 12, in this case, for example, initially across an oxidation catalytic converter 15. An injection nozzle 5 of the device 1 projects into a section of the exhaust system 3 which is located downstream of the oxidation catalytic converter 15. A reactant introduced into the exhaust system 3 there is at least partially pyrolyzed as a result of the hot exhaust gas. In order to provide a complete conversion of the urea, it is optionally possible for a mixer 16 to be provided which further mixes the hot exhaust gas with the injected liquid, and thereby further assists the pyrolysis. A hydrolysis catalytic converter 17 (which is optional, if appropriate), in which remaining urea constituents are converted into ammonia is provided downstream of the mixer 16 in the flow direction 12. Ammonia produced in this way finally impinges together with the exhaust gas on an SCR catalytic converter 13, in which the nitrogen oxides are selectively reduced.

The device 1, which is illustrated on the right-hand side of the FIGURE, has a tank 4 with a reactant 2 being shown at the top right of the tank 4. A urea-water solution is used, in particular, as the reactant 2. A line 6 leads from the tank 4 to the injection nozzle 5. The line 6 is formed with a heater 19, for example a heatable hose, in a section between the tank 4 and a pump 7. An additional filter 20, in which any impurities from the tank 4 are retained, is provided between the pump 7 and the tank 4. A partial section of the line 6 formed between the pump 7 and the injection nozzle 5 is regularly operated with elevated pressure during operation of the device 1. In this case, a sensor 18 for determining the pressure in the line 6 is also provided between the injection nozzle 5 and the pump 7. A controllable valve 14, which is likewise provided in that region of the line 6, is used to conduct a flow of the reactant 2 out of the line 6 from the section between the injection nozzle 5 and the pump 7, back into the tank 4 through a return line 8. The valve 14 is conventionally used for releasing pressure from the line 6 when the device is at a standstill, or else for circulation in the method according to the invention.

If it is established through the use of a controller 9 and the sensor 18 that gas bubbles and/or frozen urea-water solution are present in the line 6, in particular in the section between the injection nozzle 5 and the pump 7, the valve 14 is opened and the reactant is conveyed from the tank 4 through the line 6, through the filter 20 to the pump 7, onward to the valve 14 and through the return line 8 back into the tank 4. That circulation is preferably carried out until the line 6 has been flushed-through two to five times. The valve 14 is then closed again and the desired pressure is built up upstream of the injection nozzle 5 through the use of the pump 7. Those processes are controlled through the use of the controller 9, which consequently communicates with the injection nozzle 5, the valve 14, the sensor 18 and the pump 7.

Finally, it should also be pointed out that slight deviations from the system illustrated herein do not lead away from the operating method proposed herein. In particular, the position of the sensor 18, of the valve 14 or of the return line 8 may be modified, for example by virtue of the position between the pump 7 and the injection valve 5 being varied. The controller may also communicate with further sensors, for example sensors which interact with the tank and/or the internal combustion engine, and thereby generate control signals for the components mentioned herein.

At any rate, a method is specified herein which ensures the reliable operation and precise provision or dosing of reactant into an exhaust system, and consequently low pollutant emissions to the environment during operation of the motor vehicle. 

1. A method for operating a device for providing a liquid reactant to an exhaust system, the method comprising the following steps: supplying the liquid reactant in the device from at least one tank through a line to an injection nozzle leading to the exhaust system; conveying the liquid reactant through the line with a pump; returning the liquid reactant from the line to the tank through a controllable return line connected between the pump and the injection nozzle; controlling operation of the injection nozzle, the pump and the return line with a controller; and only intermittently circulating the reactant through the controllable return line.
 2. The method according to claim 1, which further comprises circulating the reactant for a conveying time period corresponding to at least two times a time required for the device to convey the reactant from the tank through the controllable return line and back into the tank.
 3. The method according to claim 1, which further comprises circulating the reactant for a conveying time period corresponding to at most five times a time required for the device to convey the reactant from the tank through the controllable return line and back into the tank.
 4. The method according to claim 1, which further comprises initiating the circulation of the reactant by using a device for identifying at least solid matter or gases in the reactant.
 5. The method according to claim 1, which further comprises initiating the circulation of the reactant at a start of operation by using a device for identifying a tank state variable.
 6. The method according to claim 1, which further comprises circulating the reactant at a conveying rate being greater than when the reactant is conveyed for injection.
 7. The method according to claim 1, which further comprises conveying a liquid urea-water solution as the reactant.
 8. A motor vehicle, comprising: an internal combustion engine; an exhaust system defining an exhaust gas flow direction from said internal combustion engine, said exhaust system including an SCR catalytic converter and an injection location for injecting a liquid reactant into said exhaust system upstream of said SCR catalytic converter in said flow direction; and at least one device including at least: a tank for the liquid reactant; a controllable injection nozzle at said injection location; a line interconnecting said tank and said injection nozzle; a pump for conveying the reactant from said tank to said injection nozzle; a return line connected to said line at a location between said pump and said injection nozzle, said return line leading to said tank; a valve for controlling said return line; and a controller for controlling operation of said injection nozzle, said pump and said return line by carrying out the method according to claim
 1. 